Ultralow-noise photonic microwave synthesis using a soliton microcomb-based transfer oscillator

Nat. Commun. 11, 374 (2020) · DOI: 10.1038/s41467-019-14059-4

License: CC BY 4.0.

Abstract

The synthesis of ultralow-noise microwaves is of both scientific and technological relevance for timing, metrology, communications and radio-astronomy. Today, the lowest reported phase noise signals are obtained via optical frequency-division using mode-locked laser frequency combs. Nonetheless, this technique ideally requires high repetition rates and tight comb stabilisation. Here, a microresonator-based Kerr frequency comb (soliton microcomb) with a 14 GHz repetition rate is generated with an ultra-stable pump laser and used to derive an ultralow-noise microwave reference signal, with an absolute phase noise level below −60 dBc/Hz at 1 Hz offset frequency and −135 dBc/Hz at 10 kHz. This is achieved using a transfer oscillator approach, where the free-running microcomb noise (which is carefully studied and minimised) is cancelled via a combination of electronic division and mixing. Although this proof-of-principle uses an auxiliary comb for detecting the microcomb’s offset frequency, we highlight the prospects of this method with future self-referenced integrated microcombs and electro-optic combs, that would allow for ultralow-noise microwave and sub-terahertz signal generators.

Figures

14 panels with data across 5 figures. Each panel page shows the plot, its columns and its files; each data.csv begins with a header naming the paper, the panel, the source, the license and the provenance route.

Fig. 1

Illustrative figure, no extractable data. Shown in the paper PDF.

Fig. 2

• panel (b): Optical spectrum of the soliton-based Kerr comb, prior to pump suppression: power (dBm) against wavelength (nm). The print gives the vertical scale as 20 dB per division, without numbers. data.csv
• panel (c): Radio-frequency spectrogram of the Kerr comb repetition rate while the auxiliary comb repetition rate is stepped: RF power (dBm, colour) against $f_{\mathrm{rep}}^{\mathrm{aux}} - 251.66$ MHz (Hz) and $f_{\mathrm{rep}}^{\mathrm{K}} - \mathrm{CF}$ (kHz), with CF = 14.092943 GHz. The print gives the colour scale as 20 dB per division, without numbers; its arrow and label are not reproduced. data.csv

Fig. 3

• panel (a): Absolute SSB phase noise (dBc/Hz) of the 14.09 GHz signal against offset frequency (Hz, log axis): the Kerr comb transfer oscillator output (blue), the Kerr comb repetition rate (green), the limit inferred from the ultra-stable laser (red) and the sensitivity limit of the phase noise analyser (grey area). The dashed line is the shot noise level. The Canceled noise arrow is not reproduced. data.csv
• panel (b): Fractional frequency change (%) of the generated RF signal for a small variation of the repetition rate, against the division ratio of the chain $N$: measured values (dots) and linear fit (solid line). Dashed lines mark zero and the zero crossing of the fit, labelled $N \approx 13{,}698$ in print. The print spaces the axis evenly in $1/N$; here it is linear in $N$, high to low as printed. data.csv
• panel (c): RF spectra of the Kerr comb repetition rate (green) and of the optical-to-microwave frequency division result (blue): power relative to the peak of each trace (dB) against frequency offset (Hz), resolution bandwidth 5 Hz. The print gives the vertical scale as 20 dB per division, without numbers. data.csv
• panel (d): RF spectrum of the frequency-divided output signal: power (dB) against frequency offset from the carrier at 14.0919 GHz (Hz), resolution bandwidth 100 mHz. The print gives the vertical scale as 20 dB per division, without numbers. data.csv

Fig. 4

• panel (a): Soliton spectrum for the lower coupling case (detuning 10 MHz): power (dBm) against wavelength (nm). The print gives the vertical scale as 10 dB per division, without numbers. data.csv
• panel (b): Evolution with detuning (MHz) in the lower coupling case: repetition rate shift (kHz, left axis, blue line with dots), soliton recoil $\Omega/2\pi$ retrieved by fitting the optical spectrum (GHz, right axis, red), and the residual repetition rate change after subtraction of the recoil induced shift (blue crosses) with its linear fit (dashed line). data.csv
• panel (c): Soliton spectrum for the larger coupling case (detuning 10 MHz): power (dBm) against wavelength (nm). The print gives the vertical scale as 10 dB per division, without numbers. data.csv
• panel (d): Evolution with detuning (MHz) in the larger coupling case: repetition rate shift (kHz, left axis, blue line with dots), soliton recoil $\Omega/2\pi$ retrieved by fitting the optical spectrum (GHz, right axis, red), and the residual repetition rate change after subtraction of the recoil induced shift (blue crosses) with its linear fit (dashed line). data.csv

Fig. 5

• panel (a): Repetition rate relative to its value at the first detuning (Hz, left axis, blue circles) and optical phase modulation to RF phase modulation conversion coefficient (dB, right axis, red crosses) against detuning (MHz). The two dashed vertical lines mark the operating points 1 and 2 of the print; its circled numbers are not reproduced. data.csv
• panel (b): Phase noise spectra (dBc/Hz) of the soliton repetition rate at the two operating points of panel (a), 1 (purple) and 2 (orange), against offset frequency (Hz, log axis). The solid black line is the laser noise; the dotted black line is the laser noise scaled down to match the 9 kHz phase calibration tone. The circled numbers of the print are not reproduced. data.csv

Fig. 6

• panel (a): Relative intensity noise RIN (dBc/Hz) against frequency (Hz, log axis): optical RIN of the pump laser (green) and microwave amplitude noise of the soliton repetition rate (purple). The dotted red line is the CW shot noise level. data.csv
• panel (b): SSB phase noise (dBc/Hz) against frequency (Hz, log axis): the repetition rate in the quiet point (blue), the limitation estimated from the pump laser RIN (green) and the estimated AM-to-PM conversion in the photodiode, the microwave amplitude noise scaled by $-25$ dB (grey). The dotted red line is the CW shot noise level. The arrows and labels of the print are not reproduced. data.csv

Cite

Erwan Lucas, Pierre Brochard, Romain Bouchand, Stéphane Schilt, Thomas Südmeyer, Tobias J. Kippenberg. Ultralow-noise photonic microwave synthesis using a soliton microcomb-based transfer oscillator. Nat. Commun. 11, 374 (2020). https://doi.org/10.1038/s41467-019-14059-4

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